Radio receiving system



July 20, 1937.

RLEAMPLkF/ER (BRO/7D) V. D. LANDON RADIO RECEIVING SYSTEM Filed June 29, 1935 Witness:

4 Sheets-Sheet l IN VENTOR VrnonllLandon ATTORNEY July 20, 1937.

v. D. LANDON RADIO RECEIVING SYSTEM 4 Sheets-Sheet 2 EL. um 9r w mum Fuuz.

. Y W 5 8 1 v 7 INVENTOR Vernon D.Landon RNN Filed June 29,

H'I'TOHNEY RADIO RECEIVING SYSTEM Filed June 29, 1955 4 Sheets-Sheet 3 INPUT 1/01. 7'6. TO 40010 13 7 183 201 185 05715670:

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i i +3 +3 a 9 an a 90 180 INVENTOR Witness Vernon llLandon July 20, 1937. v, D, LANDGN 2,087,288

RADIO RECEIVING SYSTEM Filed June 29, 1935 4 Sheets-Sheet 4 INVENTOR Vernon D.Landon HT'TORNE Y Witness:

Patented July 20, 1937 UNITED STATES PATENT OFFICE RADIO RECEIVING SYSTEM tion of Delaware Application June 29, 1935, Serial No. 28,986

12 Claims.

The present invention relates to radio receiving systems, and has for its primary object to provide an improved radio receiving system wherein noise resulting from the reception of static discharges and other interference waves, may be reduced within the receiving system to relatively low amplitudes sufiiciently below the signal level to render the interference therefrom less objectionable.

More particularly, it is an object of the present invention to provide an improved noise reduction system for radio receiving apparatus and the like, including an improved limiter circuit so arranged in the receiving system that the 15 noise reduction is most effective with the least additional apparatus.

It is also a further object of the present invention to provide a radio receiving system, the high frequency signal circuits of which are arranged in predetermined relation to the limiter circuit or circuits and provide predetermined degrees of selectivity with respect to each other.

Heretoiore it has been considered necessary to provide a series of highly selective circuits in radio receiving systems in connection with noise limiting devices and circuits. However, in accordance with the invention, it has been found that certain of the receiving circuits may be rela-- tively broadly tuned with increased reduction in the noise level with respect to the signal level, and this is attributed in part to the fact that static and other interference impulses, while of relatively low amplitude in sharply tuned circuits, may persist as oscillations over a relatively 5 long time interval in such circuits. It has been found that in more broadly tuned circuits, the amplitude of the interference waves may reach higher levels but may be attenuated more rapidly. In conjunction with an improved limiter circuit or circuits, a system may be devised which provides much greater improvement in noise reduction than would be expected from mere amplitude limitation. That is, the noise is not merely limited, to the same audio frequency level as the signal, but is reduced to a value far below the signal level.

A limiter system of the type to which the invention relates is shown in the patent to Langmuir #l,468,116 wherein a current limiting device which will eliminate all of the current impulses above a predetermined value is employed to reduce heavy static discharges and, after the large impulses have been removed, suitable tuning apparatus is employed to select the signals 55 of the frequency to be received.

for the elimination of static and other undesired impulses tending to create noise in the output of a radio receiving system.

The invention will, however, be better understood when considered in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Referring to the drawings, Figures 1 and 1A jointly show a schematic circuit diagram of a radio receiving system embodying the invention;

Figs. 2 and 3 show curves illustrating certain operating characteristics of a limiter circuit embodied in the system of Fig. 1;

Fig. 4 shows mathematically derived decay trains for impulses or oscillations applied to two stage amplifiers each having four tuned circuits coupled in groups of two with critical coupling, and illustrating the operation of a receiving system as shown in Fig. 1, and v Fig. 5 is a schematic circuit diagram of a limiter circuit embodying the invention, being a modification adapted for use in the signal amplifying channel of a radio receiving system.

Referring to Fig. 1, the receiving system shown is, by way of example, of the superheterodyne type and is also arranged for multi-range signal reception. A radio frequency amplifier 5 and a combined detector-oscillator or mixer tube 1, are provided with suitable tuning capacitors 9 arranged to be connected with interstage coupling transformers and inductances indicated at l l and i3 for the short wave and broadcast bands, respectively, through a gang wave-changing switch comprising selector sections indicated at and antenna section I6.

The circuit described represents any suitable high frequency radio receiving circuit comprising a series of tuned circuits in conjunction with suitable amplifier electric discharge devices. As such systems are well known and understood further description is believed to be unnecessary.

In the present example, the receiving circuit provides an intermediate frequency signal which is delivered to the terminals ll of a multi-stage intermediate frequency amplifier comprising a series of tuned coupling transformers l9, 2|, 23 and 25 for high gain electric discharge amplifier tubes 21, 29 and 3| forming the first, second and third intermediate frequency amplifier stages, respectively.

The tuned coupling transformer 23 may be considered as the input transformer for a combined limiter and amplifier unit or stage comprising a. fourth intermediate frequency amplifier stage, including an amplifier tube 33, and a limiter circuit comprising a limiter tube, 35 of the electric discharge type.

In the present example, the limiter tube 35 is of the screen-grid pentode type having a control grid 31 connected to ground 4I through the tuned.- secondary 33 of the coupling transformer 25. The tube is further provided with an indirectly heated cathode 43 also connected to I ground 45 through a variable resistance device 41 shunted by a suitable high frequency by-pass condenser 43. x

The tube 35 is further provided with a. screen grid III which receives operating potential through a lead 53 from a tap 55 one potential supply resistor 51 of a power supply unit 53 of the rectified alternating current type. One end 6| of the supply resistor 51, is grounded and the polarity of the direct current supply is indicated at the terminals thereof. A suppressor grid connected tothe cathode is indicated at 63, and the output anode of the device 35 is indicated at 85.

The limiter circuit further includes coupling means for the intermediate frequency amplifier tube 33, comprising an anode circuit coupling resistor 61 of relatively low resistance value connected through a lead 63 with a lower positive potential tap 1I than the tap 55 for the screen grid on the supply resistor 51. The anode circuit is thus completed from the anode through the coupling resistor 61 to the tap 1I thence through the potential supply section included between the terminals H and 6|, and to the cathode through the self biasing control resistor 41. The potential drop in the latter supplies the biasing potential to the control grid 31.

The coupling with the fourth intermediate frequency amplifier tube 33 is completed through a coupling condenser 13 to the control grid 15 of the tube 33 and biasing potential is supplied to said grid through a grid leak resistor 11 grounded as indicated at 13. The biasing potential for the grid 15 is supplied from a self bias source comprising a variable self bias resistor 8| grounded as indicated at 83 and connected with the cathode of the clevice33 indicated at 85. The variable self bias resistor is provided with a suitable high frequency by-pass condenser 81 in shunt therewith.

The tuned signal receiving circuits up to the limiter circuit including the device 35 are relatively broadly tuned. As a practical circuit arrangement in the present example, the radio frequency amplifier may be made to respond broadly to signals at any frequency adjustment in a band preferably 50 kilocycles wide, this being within the acceptance band limit for the audio frequency side band requirements.

The intermediate frequency amplifier circuits associated with the transformers I9, 2I 23 and 25 are more broadly tuned, and it has been found that at present, the acceptance band of the intermediate frequency amplifier or band pass characteristic may be increased to 50 kilocycles in width. However, with. a further band increase, there is a further improvement in the noise reduction in conjunction with the limiter circuit following it, when the band width is increased up to 500 kilocycles.

Following the limiter circuit which is inthe broad band pass intermediate frequency amplifier, the signal circuits are sharply tuned to narrow band pass limits prior to the second .or audio I frequency detector. In the present example, the

former 95. The circuits are preferably critically coupled and tuned to the exact intermediate frequency within relatively narrow limits of band acceptance. While one pair of tuned coupled circuits is shown, others may be employed after the manner of the preceding circuits, each'of the tuned circuits being sharply tuned to the desired intermediate frequency. The purpose of the sharply tuned circuits following the limiter circuit and preceding the audio frequency detector will hereinafter be described.

In the present system, the audio frequency detector 83 is a tube of the double diode pentode type, the diodes 31 being connected in parallel as a single diode element, to the high potential side or lead 33 of the secondary tuned circuit 33, and the cathode being connected to the opposite side or lead IOI through a suitable filter resistor I33 provided with by-pass capacitors I05 to the cathode, and a diode rectifier output resistor I 01, the return circuit to cathode being taken through the cathode return lead T33, thereby completing the diode rectifier circuit. A self bias resistor III is included in the' cathode return lead I03 between the diode circuit connection H3 and ground connection II5.

Audio frequency signals are conveyed to the pentode amplifier portion of the detector through a movable volume control contact I I1 on the diode rectifier output resistor I31, thence through a coupling condenser II3 to the grid input circuit lead I2I. The detector is provided with the usual screen and suppressor grids I23 and I25 respectively, and the output anode I21 is coupled through a suitable coupling network I23 with an output amplifier tube I 3I. Theoutput tube I3I is coupled to a loudspeaker or other suitable output device I33 through an output coupling transformer I35.

The audio frequency detector is preferably of the diode rectifier type, as shown, and may be coupled to any suitable audio frequency amplifier represented by the pentode portion of the detector 33 and the amplifier tube I3I. As the audio frequency amplifier does not directly concern the invention, further description thereof is believed to be unnecessary. It will be noted, however, that the anode potentials for the audio frequency amplifier as well as the'anode and screen grid potentials and the remainder of the system are taken through suitable leads I31 and I33 from the common power supply means 53.

It will further be noted that the receiving system is provided with a sensitivity control means in the form of a variable cathode lead bias resistor I4I representing any suitable sensitivity control means for the receiving system, and, in the present example, is included in circuit with a cathode lead I43 for the radio frequency amplifier tube 5, thereby to control the gain of that tube by varying the control grid bias thereon.

n An

The sensitivity control means preferably isaramplifying channel preceding the limiter circuit. Furthermore, automatic volume control means, indicated at I45, may be coupled through a lead I" with a portion of the amplifying channel, preferably in conjunction with the intermediate frequency amplifier, to provide automatic volume control potentials through control leads indicated at I49 and I5I, further controlling the radio frequency amplifier and intermediate frequency amplifier stages in any suitable manner, for automatic volume control purposes. The automatic volume control means is provided with a separate amplifier channel including an amplifierportion of the tube I coupled with a rectifier portion including the diodes I44 and a cathode I46, through a coupling transformer I48.

The automatic volume control means controls the amplification or sensitivity of the system in circuits preceding the limiter circuit, and is provided for impressing constant signal level on the limiter circuit.

The selectivity of the receiving system may be adjusted, preferably in one or more of the intermediate frequency amplifier circuits. In the present example,-selectivity adjustment means is provided in the input circuit of the first intermediate frequency amplifier 21 and comprises a coil I53 inductively coupled with the primary winding of the transformer I9 and connected in series with the secondary thereof through a selector switch I55. The switch provides connection with a contact point I51 when moved from the normal position shown, to include the coupling coil I53 and a resistor I59 in the tuned secondary circuit, thereby increasing the coupling and decreasing the selectivity of the amplifier input circuit.

When the switch is in the position shown, the selectivity is normal and may be the same as in the succeeding amplifier stages for any preselected side band acceptance, such, for example, as between to 500 kilocycles, as is at present preferred, depending upon the chosen intermediate frequency.

The intermediate frequency amplifier stage preceding the limiter circuit is provided with a limiter level control means comprising a variableself-bias resistor IGI in the cathode circuit I63 of the intermediate frequency amplifier tube 3|, the arrangement being such that the control grid bias may be varied thereby, independently of and in addition to the bias potentials supplied by the automatic volume control means I45. By operation of the control device IBI, the gain of the intermediate frequency amplifier next preceding the limiter circuit may be varied to control the signal level applied to the limiter. The automatic volume control holds the level constant for all signals. This control varies the level to which signals are held.

Separate control of the limiter action is provided by the variable resistor 41, whereby the grid bias potential on the limiter tube 35 is adjusted. In the signal channel, the fourth intermediate frequency amplifier 33 variable self-bias resistor 8| provides additional gain controlling means following the limiter circuit and preceding the second or audio detector. 7

Briefly stated, the receiving system includes a signal limiter device, preferably of the type shown, with a tuned signal amplifier having relatively broad band pass characteristic preceding the limiter device, and a tuned signal amplifier following the limiter device, said last named amplifier having a relatively narrow band pass characteristic, with automatic volume control means for providing signals of substantially constant amplitude to the limiter device.

The limiter circuit depends upon plate voltage saturation for limiting the positive grid part of the cycle, and plate current cut-off for limiting the negative part. In the circuit shown, the anode supply lead 69 for the limiter tube 35 is connected to a lower potential supply point than the screen grid, and with a tube such as that indicated at 35, which maybe a tube known commercially as the RCA 606, 30 volts at H and 90 volts at and with substantially 3 volts negative bias on the grid 31 has been found to be satisfactory for operation of the device in the limiter circuit.

In operation, the limiter circuit should be operated with about 1 volt R. M. S. signal applied to its grid and hence it should, as shown, be preceded by an amplifier ii Weak signals are to be received. In the present example, it is preceded by both the radio frequency and intermediate frequency amplifiers to insure the required strength of signal. The signal level or R. M. S. voltage applied to the limiter may be adjusted to substantially 1 volt by means of the limiter control means I6I.

Referring now to the curves shown in Figs. 2 and 3, the curve I plotted between plate current/and bias voltage on the limiter tube 35 indicates the direct current operating characteristic of the limiter circuit.

The desirable characteristic curve of the limiter consists of a steep straight operating portion terminated abruptly at each end by a horizontal portion. This is because the signal is to swing the grid over the steep portion without excessive distortion, while larger impulses swinging the grid over the fiatportions of the curve, cause only slightly greater fluctuations in output than the signal itself. To obtain this type of curve the voltages given are used, whereby the steep operating portion of the curve is abruptly terminated in the region of 1 volt bias by anode voltage saturation and in the region of 5 volts bias by anode current cut-off. The same type of saturation may be had at lower signal levels by using lower screen and plate voltages and a lower bias voltage. Also raising these voltages gives a limiter which limits at higher signal levels.

The plate resistor should not be made too large or the capacity. of the tube will prevent the R. F. currents from following this D. C. curve.

The curve I61, shown in Fig. 3, is plotted between signal input volts on the preceding amplifier stage 3| and the output volts derived from the limiter output circuit as applied to the fourth stage intermediate frequency amplifier 43.

Control 41 is used to adjust the bias of the limiter to the center of-the operating portion of the curve I65 of Fig. 2.

The abscissa of curve I61 of Fig. 3 is the voltage applied to a preceding amplifier grid. The voltage applied to the limiter itself is about 2 volts at the knee of the curve or 1 volt half way up the straight portion. Thus if 1 volt is applied there is space for modulation on the straight part of the curve. If the modulation seldom reaches 100%, slightly more carrier may be applied.

The main selectivity of the receiving system is obtained from a circuit or circuits following the limiter. However, sufficient selectivity must be provided in advance of the limiter to reduce However, the amplitude of the response to an impulse varies directly with band width. Thus the area under the envelope of the impulse train is independent of the band width and this and the above facts will more readily be seen from a consideration of the representation of various decay trains as shown in Fig. 4.

The trains are plotted against time on the horizontal axis, and the total length of time traversed by the entire width of the graph from the left hand margin to the right hand margin is 260 micro-seconds as indicated on the diagram. The

heavy oscillatory graph A represents the decay of a wave train in an amplifier having two stages, each having two tuned circuits, such as the circuits connected with the amplifier tubes 21 and 29 in Fig. 1.

The decay train shows rapid attenuation and ends abruptly at A1. as indicated, with the first impulse A: foreshortened in the graph representation, as it extends normally substantially times the height shown and, therefore, could not well be drawn in the same proportion in the space shown. The acceptance band width of the tuned circuits for the diagram A is substantially 500 kilocycles.

The dotted graph B of a decay train starting at the zero point indicated for the graph A and extending to the point B1, represents an acceptance band width of 50 kilocycles.

When the amplifier circuits are sharply tuned to an acceptance band width of 5 kilocycles, a wave train generated by an impulse may have the form shown by the graph C which startsfrom the same zero point indicated on the diagram and may extend well beyond the timelimit indicated between the right and left hand border-lines oi the diagram.

It can, therefore, be seen from the oscillation diagrams or graphs of Fig. 4 that the length of the decay train may be inversely proportional to the acceptance band width of the amplifier and that the more broadly tuned circuits serve more rapidly to attenuate the oscillations than the more selective circuits although the amplitude is relatively highin the broader tuned circuits.

It can, therefore, be seen that in accordance with the invention, the opportunity for improve-.

ment in noise suppression is much greater than would be expected from mere amplitude limitation. That is, the noise voltage amplitude may not merely be limited to the sameaudio frequency level as the signal, but may be reduced to a value far below the signal level.

This is accomplished by creating in the preceding amplifier tuned circuits, 9. condition of operation represented by the curve A, for example, or the curve B to provide a short, high amplitud impulse, and then reducing the amplitude of the impulse by a limiter circuit of the type shown in Fig. i in connection with the tube 35 to a limit such as indicated between the lines L in Fig; 4,

ing sharply tuned transformer 95, following the amplifier 33 which is directly coupled to the limiter circuit. In this manner, the shortened impulse is then extended in much the same manner as the impulse of the curve A is extended to the curve C and its amplitude is in the same proportion reduced to a value far below that of the signal.

It can, therefore, be seen that it is desirable,

to supply tuned circuits in advance of the limiter circuit of sufiicient selectivity to prevent unnecessary interference between signal channels and then to provide sufilcient broadly tuned circuits to permit a short decay train before entering the.

limiter circuit. The amplitude of the decay train is then limited to a predetemined low value such operation is maintained between all local and semi-local broadcasting stations. In view of this,- it seems logical to utilize a 50 kilocycle acceptance band width in order to obtain an appreciable attenuation at 50 kilocycles from the center of the band. The receiving system shown in Fig. 1 employs an intermediate frequency centering on :425 kilocycles and the acceptance band is.50 kilocycles wide, up to the limiter circuit.

It is desirable that substantially constant intermediate frequency voltage be maintained on the grid of the limiter tube and, for this reason, it is desirable to employ a substantially fiat automatic volume control means such as that indicated in the circuit of Fig. 1.

Further referring to the circuit of Fig. 1, in the broadcast band indicated by the letters, B. C. resistors indicated at I69 are employed to maintain a 50 kilocycle band width.

Referring now to Fig. 5, there is shown a static or noise limiter circuit embodying the invention and adapted for insertion in a signal channel of a radio receiving system, preferably immediately preceding the second audio frequency detector.

The circuit is associated with a signal amplifier circuit comprising an electric discharge amplifier tube "I which may be of the high mu type comprising a cathode I13, an output anode I15, sup-.

pressor grid I'Il connected with the cathode, a control grid I19 adjacent to the cathode and a screen grid I8I between the control grid and the anode I15.

The amplifier stage is provided with input and output coupling means represented by the input transformer I83 and the output transformer I85. The primary and secondary windings of the transformers are tuned by suitable condensers I81 and I89 to the selected signal frequency to b transmitted by the amplifier stage. The two coupled circuits provided by the input coupling means for the amplifier iinit are broadly tuned by the adjustable condensers I8'I for a relatively wide acceptance band width including the selected frequency, while the output coupling means is sharply tuned by the condensers I89 which are preferably variable as indicated for this purcircuits of the input coupling means may be tuned to substantially a 500 kilocycle acceptance band width or 250 kilocycles on either side of the selected frequency of 425 kilocycles.

The output coupling means for the limiter and amplifier unit, on. the other hand, may then be tuned for acceptance band width from 5 to 10 kilocycles to provide maximum selectivity immediately following the amplifier stage and preceding the audio detector.

The amplifier tube III is not directly connected to the input circuit or input coupling means but, instead, a limiter tube I9I is interposed between the tuned input circuit, which, in this case, is the tuned secondary circuit I93 of the transformer I83, and the input I95, I91 of the amplifier tube Ill.

In the present case, the input circuit of the amplifier tube comprises the lead connected with the control grid I19 and the grid leak resistor I9'l through which bias potential is supplied to the grid. The operating potentials and values therefor are indicated at the terminal ends of the various leads.

A coupling means having uniform frequency response is provided between the limiter tube I9I and the input circuit I95, I91, and, in the present example, is provided by a coupling resistor I99 as an output impedance coupling element in the output circuit 20I of the limiter. The coupling between the two circuits is provided by a coupling condenser 293 connected between the high signal potential sides of the circuits 2M and I95, as

- shown.

The limiter tube I9I includes a cathode I92, a control grid I94, a screen grid I96, a suppressor grid I98 and an output anode 200. The device is of the pentode amplifier type and it will be noted that the suppressor grid is connected with the cathode and that the anode is arranged to receive a relatively low positive potential of substantially 30 volts with respect to the positive potential applied to the screen grid, of 90 volts. With this arrangement, the operation of the limiter tube depends upon the plate voltage saturation for limiting the positive grid voltage of the single input cycle and depends upon plate current cut-off for limiting the negative portion of the input cycle.

The limiter circuit, therefore, comprises a broadly tuned input coupling device such as the transformer I83, a sharply tuned output device such-as the transformer I85 with tuned impedance coupled pentode devices connected therebetween the first of said pentode devices being operated at a relatively low anode potential with respect to the screen grid potential. The circuit furthermore is adapted to be included in a receiving system preceding the audio frequency detector as indicated.

With this circuit arrangement the noise impulses impressed upon the input circuit of the limiter-amplifier unit or stage are of very high amplitude and small time duration, while the signals are of relatively low amplitude and of long time duration. It is these different characteristics of noise and signal which permits their separation to be accomplished in the combination limiter and amplifier stage shown in Fig. 5. The sharply tuned amplifier output circuit together with the broadly tuned limiter circuit having a coupling means between the limiter and amplifier portion with substantially a fiat frequency characteristic permits the character of the noise wave to be made more like that of the signal, that is to be changed from high amplitude to low amplitude and from small time duration to long time duration. The change in the character of the noise is inversely proportional to the amplifier band width in circuits preceding the limiter amplifier unit.

From the foregoing description, it will be seen that in accordance with the invention, a radio receiving system, preferably of the superheterodyne type, may be provided with signal amplifier circuits preferably substantially broadly tuned, and included means for changing the selectivity and gain both manually and automatically to provide at the output terminals of the broadly tuned receiver portion, a signal of substantially constant amplitude and band width and that the signals are then limited and amplified in a combined limiter and amplifier stage or unit a sharply tuned circuit supplying the limiter device and a broadly tuned circuit being provided for coupling the output amplifier device to a succeeding circuit. Furthermore, the amplifier is preferably provided with individual gain control means while potential control of the signal output of the receiving system is preferably controlled by means located in association with the audio frequency detector or amplifier.

I claim as my invention:

1. In a. radio receiving system, means for improving the signal-noise ratio thereof, comprising a signal limiter device, a tuned signal amplifier having a relatively broad band-pass. characteristic preceding the limiter device, and a tuned signal amplifier'following the limiter device, said last named amplifier having a relatively narrow band-pass characteristic. 1

2. In a radio receiving system, means for improving the signal-noise ratio thereof, comprising a signal limiter device, a tuned signal amplifier having a relatively broad band-pass characteristic preceding the limiter device, a tuned signal amplifier following the limiter device, said last named amplifier having a relatively narrow band-pass characteristic, and means providing substantially constant signal level at the limiter device.

3. In a radio receiving system, means for improving the signal-noise ratio thereof, comprising a signal limiter device, a tuned signal amplifier having a relatively broad band-pass characteristic preceding the limiter device, and a tuned signal amplifier following the limiter device, said last named amplifier having a relatively narrow band-pass characteristic, said limiter device including a signal amplifier tube having a cathode, anode, and control, screen and suppressor grids, and means for applying to said electrodes potentials for permitting anode potential saturation within the limits of signal fluctuations in one direction and permitting anode current cut-off within the limits of signal fluctuations in the opposite direction, thereby to limit said signal fluctuations.

4. In a radio receiving system, means as defined in claim 3, further characterized by the fact that means are provided for adjusting the operation of said limiter device for limiting the signal amplitude variations to a predetermined level.

5. In a radio receiving system, means for improving the signal-noise ratio thereof, comprising a signal limiter device, a tuned signal amplifier having a relatively broad band-pass characteristic preceding the limiter device, a tuned stantially constant signal level at the limiter device, and means for varying the level at the limiter device to which the signals are held by said last named means.

5 6. In a radio receiving system, a signal amplifier unit including a tuned input circuit, an electric discharge amplifier device connected therewith, a second electric discharge amplifier device, means providing an output circuit for said second amplifier device relatively sharply tuned with respect to said input circuit about the same mean frequency, means having substantially uniform frequencyresponse coupling said amplifier devices in cascade relation, and means for operating one of said devices as a limiting means for signals impressed upon said input circuit by anode voltage saturation and anode current cut-01f.

7. In a radio receiving system, a signal amplifier unit including a tuned input circuit, a pentode electric discharge amplifier device connected therewith, a second pentode amplifier device, means providing an output circuit for said second amplifier device relatively sharply tuned with respect to said input circuit about the same mean frequency, means having substantially uniform frequency response coupling said amplifier devices in cascade relation, and means for operating one of said devices at a relatively low anode potential with respect to a screen grid potential thereby to provide limiting means for signals impressed upon said input :ircuit by anode voltage saturation and anode current cut-off.

8. In a radio receiving system,'a signal amplifier unit including a tuned input circuit, an

5 electric discharge amplifier device connected therewith, a second electric discharge amplifier device, means providing an output circuit for said second amplifier device relatively sharply tuned with respect to said input circuit about the 40 same mean frequency, means having substantially uniform frequency response coupling said amplifier devices in cascade relation, and means for operating one of said devices as a limiting means for signals impressed upon said input circuit by anode voltage saturation and anode current cut-off, and means for impressing signals of substantially constant amplitude on said input circuit.

9. In a. superheterodyne receiver, means providing relatively broadly tuned high frequency and intermediate frequency amplifiers, automatic ing a pair of resistance coupling pentode electric discharge amplifier devices, and means for applya limiter device depending upon plate potential 10 saturation for limiting the positive portion of a' signal cycle and upon anode current cut-oil for limiting the negative portion of said cycle.

10. In a superheterodyne receiver, an output intermediate frequency amplifier unit comprising 20 I a pair of resistance coupled pentode electric discharge amplifier devices, and means for applying potentials-thereto whereby one of said devices may effectively amplify signals and the other of said devices may effectively limit the amplitude 25 of said signals to a predetermined level.

11. In a superheterodyne receiver, an output intermediate frequency amplifier unit comprising potentials thereto whereby one of said devices may eiiectively amplify signals and the other of said devices may efiectively limit the amplitude of said signals to a predetermined level, said intermediate frequency amplifier unit having a relatively broadly tuned input circuit and a relatively sharply tuned output circuit, and means for controlling an operating potential of at least one electrode of each of said devices thereby to control the gain and the limit of the signal ampli- 40 tude of signals transmitted through said unit.

12. In a radio receiver, a noise limiting system comprising a limiter device, tuned s gnal circuits having a broad band pass characteristic at least 50 kilocycles in width for supplying signals to said limiter device, a signal amplifier coupled to said limiter device and having a tuned output circuit having a relatively narrow band pass characteristic less than kilocycles in width.

VERNON D. LANDON. 

